Two Cheers for GitHub
To understand why, you need to understand how Facebook plugins work technically and socially. Nobody actually plays Scrabble "on" Facebook; instead, when you click on the tiles to put down the word "syzygy", Facebook sends a packet of information from its server to the server that hosts the Scrabble game. That server calculates your score and gives you new tiles and then produces a chunk of HTML that it sends back to Facebook. Facebook then inserts that HTML into its own page to display to you.
The key thing here—the thing that fuelled Facebook's explosive growth—is that Facebook doesn't have to write, debug, and maintain the Scrabble program. They didn't have to create ZooVille, either, or the appointment calendar app my saxophone teacher used to use, or anything else. Instead, Facebook provides a platform that keeps track of users' passwords, their connections with each other, and (crucially) what plugins they've enabled. And this has meant that Facebook's users can take advantage of the work of millions of programmers, rather than "just" what a few thousand Facebook employees are able to produce.
Imagine what the world would look like if GitHub worked like this. Imagine a world in which a grad student with time on her hands could have built a CSV display widget and offered it to the GitHub community two or three years ago. Imagine a world in which a research group that had built performance prediction tools for scientific software could wrap those up as a plugin so that people could browser for hotspots in their R code as they were doing code reviews. Imagine—
What's that? GitHub has an API? Why yes, that's true, and in theory that means people can provide all these services right now. The problem, though, is that if people have to leave one site and go to another in order to use your software, they won't. It's like asking people to go up a single flight of stairs in order to collaborate with one another: it doesn't seem like a big deal, but it cuts communication by more than 90%. And if they have to download and install something, well, how much fun has that ever been?
Facebook isn't our only guide to how much better life would be if GitHub was a platform rather than a closed box. In the early 2000s, Eclipse was the IDE of choice for millions of programmers. A major reason for its popularity was its plugin ecosystem: if you had a new code analysis package, or a better interface to a web-hosted bug tracker, you could offer people just that without having to design, build, or test all the other things needed to make a tool credible. (As a colleague of mine said back then, "Eclipse means I never have to figure out how to get stuff to print again.")
Science is huge: it encompasses everything in the universe. But it's also pretty small: there are often only a handful of people who really understand some topic, and only a handful more who even know it exists. Software companies can't afford to serve that long tail themselves, but they can give us the means to help ourselves. It does mean giving up a bit of control, but it doesn't seem to have hurt Facebook or Eclipse. So, two cheers once again for GitHub, and here's hoping that we'll be able to give them a third cheer soon.
For more on this subject, see Steve Yegge's epic rant about Google+. And yes, Facebook is a walled garden that profits from selling people's private lives, but that doesn't undermine the arguments in favor of platforms.